The present invention relates to a bidirectional protection device and more particularly to such a device monolithically manufactured.
Such a device can be used for protecting electric or telephone lines from overvoltages by becoming conductive when the voltage at its terminals overcome a determined position or negative value.
FIG. 1 shows the current-voltage characteristic of a conventional bidirectional protection component. Considering the positive quadrant of this characteristic, when the voltage across the component reaches a value V.sub.BO1, the current increases rapidly at a substantially constant voltage. Then, abruptly, the voltage across the component drops as the current increases. The current value stabilizes as a function of the series impedance of the circuit at a value I.sub.ON for which the voltage across the component has a low value V.sub.ON. When the voltage across the component decreases, the current decreases down to a hold current value I.sub.H corresponding to a voltage value V.sub.H. Below this value I.sub.H, the component is again in the blocked state.
In the negative quadrant, the component has a similar characteristic.
There already exists numerous components having such characteristics. In the present application, one will consider a component having a low hold current, not higher than a few milliamperes, a breakdown voltage not higher than a few tens of volts, and a maximum current I.sub.BO for the breakdown voltage not higher than a few milliamperes. Moreover, it is desired to obtain a component such that the voltage drop in the conductive state, V.sub.ON, is particularly low for low current values in the conductive state I.sub.ON. It is also aimed at achieving a component in which the positive and negative breakdown voltages, V.sub.BO1 and V.sub.BO2, respectively, can be fixed independently. Such an independent setting of V.sub.BO1 and V.sub.BO2 is useful, for example, for protecting lines having different polarizations in a rated state.